Six years after Alfred Wegener published his theory on continental drift, the geophysicist scrutinized another curious relationship, one so obscure, however, that it would take almost a century to confirm his hypothesis.

During the winter of 1916–1917 in the Vosges Mountains near the French-German border, he noticed tufts of ultrafine, 4-centimeter-long strands of ice sprouting from dead branches. The first scientist to investigate the hair ice phenomenon, Wegener reported the next year a likely relationship between the projecting ice strands and fungus filaments that also caught his eye, clinging to the surface of the hair ice–bearing wood. Yet how the two were connected remained a mystery—until now.

Last week, a team of German and Swiss scientists not only identified the fungus behind these exotic ice structures, they also have explained how an interplay among biological factors, atmospheric conditions, and basic properties of liquid and frozen water may lead to the delicate tresses of hair ice. The researchers published their findings on 22 July in Biogeosciences, a journal of the European Geosciences Union.

Fungus Found

Hair ice grows under conditions of freezing temperature and high humidity. From 2012 until last year, the researchers created these conditions from time to time in a garden shed to conduct hair ice–growing experiments.

In the absence of the fungus, ice formed only a simple crust on the wood.The researchers credit the elaborate ice coiffures subsequently created to Exidiopsis effusa, from the phylum Basidiomycota. It is among 11 different species of fungi that biologist Gisela Preuß, a secondary school teacher in Neustadt, Germany, and coauthor on the report, found from microscopic examination of hair ice–growing wood.

“Exidiopsis effusa colonized all of our hair ice–producing wood, and in more than half of the samples, it was the only species present,” she said.

In the tests, killing Exidiopsis effusa with hot water prevented new hair ice from developing when favorable hair ice conditions were restored. Also, in the absence of the fungus, ice formed only a simple crust on the wood.

Ice Hairs Sprout Out

The new tests also tracked temperature variations and chemical composition of water and ice during hair ice growth. They confirmed hypotheses about how hair ice forms posed by other scientists who, since Wegener, also investigated the curious filaments. The growth of hair ice filaments is seen below in a time-lapse video taken at the Gletschergarten (Glacier Garden) in Lucerne, Switzerland. Credit: Erich Albisser/Gletschergarten Luzern CC BY-NC-SA.

Unlike the spiky ice crystal crusts, rime ice and hoarfrost, which grow under conditions similar to those of hair ice but with new crystals added at their tips, hair ice grows as its name implies: with the newest crystals forming at its roots, near the wood.

It’s there that a film of water gets trapped between wood and ice in such a way that ice hairs sprout radially from the wood surface, like spokes of a wheel, the researchers report. Suction forces within the film draw water from within the pores of the wood toward the freezing front, maintaining the sandwiched water film while adding new crystals to the hairs.

Most known observations of hair ice have come from broadleaf forests, largely located between 45°N and 55°N latitude, in regions including Scotland, Ireland, Wales, the Netherlands, Sweden, Slovenia, Russia, the U.S. Pacific Northwest, and Canada’s west coast.

Fungal Freeze Control

With or without fungal activity, “the same amount of ice is produced on wood,” said study coauthor Christian Mätzler, an emeritus professor of physics at the University of Bern, Switzerland, in a press release. Although the fungus is a key player in sculpting hair ice, the resulting fibers do not encapsulate any fungus filaments. Rather, the fungus remains on the wood as white webby films.

Instead of acting as a scaffolding for ice fibers, the fungus appears to foster hair ice growth by secreting a chemical that prevents small ice crystals from reconfiguring into larger ones—a process known as recrystallization.

“The action of the fungus is to enable the ice to form thin hairs—with a diameter of about 0.01 millimeter and to keep this shape over many hours at temperatures close to 0°C,” explained Mätzler. “Our hypothesis includes that the hairs are stabilized by a recrystallization inhibitor that is provided by the fungus.” This inhibitor, likely a protein, prevents damage that recrystallization would wreak on structures that are fine grained, such as hair ice filaments.

Janine Fröhlich of the Max Planck Institute for Chemistry in Mainz, Germany, who was not involved in the study, commented that Exidiopsis effusa’s effect on ice formation might serve as a mechanism for water collection to aid growth of the fungus. A similar process has been suggested for ice-forming lichen, she told Eos.

Chemical analyses of hair ice meltwater have provided clues to how the fungus may have inhibited recrystallization. Using mass spectrometry, chemist and study coauthor Diana Hofmann of Forschungszentrum Jülich, Germany, found organic compounds in the water, primarily fragments of lignin, which is the main component of wood and a substance that some fungi and bacteria can break down but animals can’t. The study authors say more research is needed to determine what role the decomposed lignin may play in hair ice growth and to pin down if the lignin breakdown products are serving as the recrystallization inhibitor.

Fungi apparently evolved ways to trigger ice nucleation to protect themselves from damage when temperatures drop below freezing, said atmospheric chemist Thomas Hill of Colorado State University, who did not participate in the study. “The main point is to protect the fungus by initiating freezing at relatively high temperatures,” he said.

“It’s satisfying to know the organism responsible for such a beautiful thing.”Among particles and organisms that are ice nucleators, anything that triggers water to crystallize at −5°C or warmer exhibits a rare ability in nature, Hill explained. He called Exidiopsis effusa’s apparent ability to trigger ice nucleation just below 0°C “really exceptional.”

“It’s satisfying to know the organism responsible for such a beautiful thing,” Hill continued. He added that this and other recent discoveries show that ice nucleation seems to be much more common than previously thought. However, we still know almost nothing “about what ecological advantages it provides,” he explained.

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